1. Field of the Invention
The present invention generally relates to communication systems and in particular to wireless communication systems.
2. Description of the Related Art
Communication systems, and in particular, wireless communication systems are limited in the amount of resources they can allocate to their users. Examples of resources for wireless communication systems include transmit power, bandwidth and data rate. In some wireless communication systems such as cellular Code Division Multiple Access (CDMA) systems, a user equipment (UE) is assigned to a plurality of cell sites that is referred to as that UE's active set of cells. A cell is a geographical area defined by physical boundaries. Each cell has base station equipment or a cell site that services the UEs located in that cell. A UE is serviced by a cell site when the cell site performs various operations in accordance with protocols being followed by the communication system to enable the UE to communicate with the system and/or with other user equipment of the system. Examples of user equipment include cellular telephones, pagers and wireless personal computers. A protocol is generally a set of rules that dictate how communication is to be initiated, maintained and terminated between system equipment and/or user equipment of the communication system. Protocols are usually part of a standard established by standards bodies, industry groups and/or governmental regulatory bodies.
Wireless communication systems such as systems that comply with the well known 1x-EV-DO (cdma 2001x-Evolution-Data Optimized) and 1xEV-DV (cdma 2001x-Evolution-Data Voice) standards as well as the High Speed Downlink Packet Access (HSDPA) specification in the Universal Mobile Telecommunication System (UMTS) standard can accommodate the conveyance of data information and are hereinafter referred to wireless data systems. The wireless data systems are structured in substantially the same manner as other wireless communication systems in that they comprise a plurality of base stations each located in a cell.
Base station equipment (or cell sites) is generally system equipment comprising communication equipment (e.g., radio transmitters, receivers, processing equipment) owned, controlled and operated by system providers. System providers are entities such as local telephone companies, long distance telephone companies, Internet Service Providers (ISP) and other communication service providers.
As a UE migrates between cells of a communication system, it selects a cell—from its active set of cells—to provide it service. The UE selects the cell based on C/I values it measures from all the members of its active set of cells. The C/I, which is the Carrier to Interference ratio, is calculated by the UE from the measurement of pilot signals transmitted by the cell site. Each cell site transmits a pilot signal that, inter alia, serves to indicate the presence of system equipment of a wireless communication system to UE's in the vicinity of the cell. The transmission power of the pilot signal of the cell site is known by the UE and thus the UE is able to calculate the C/I from its measurement of the power of received pilot signals. The C/I is essentially equivalent the signal-to-noise (SNR) ratio of a communication channel of the system. The servicing of a UE by a cell site is the provision of resources by the cell site to the UE to allow the UE to communicate with other users and system equipment of the communication system in accordance with a protocol being followed by the communication system.
The calculated C/I is one type of Channel Quality Indicator (CQI) that can be sent by a UE to members of its active set. The CQI is a type of information that a UE can transmit to members of its active set of cells that indicates the relative quality of the communication channels between the cells and the UE. The UE can measure and send other types of CQI information to the members of its active set. When the UE is located in the communication system such that it can be serviced by more than one cell, it has to decide from which cell it will receive service. Typically, the UE selects the cells with the best channel quality indicated by CQI among its active set of cells. For example, a UE would select the cell site with the highest C/I among its active set of cell sites and switches to that cell site, i.e., obtains service from that cell site. The UE selects a cell site and thus switches to that cell site by transmitting CQI information that can only be decoded by that cell site. Thus, for example, in CDMA communication systems, the UE encodes its CQI information with the Walsh cover of the selected cell site thereby enabling only the selected cell site to decode the transmitted CQI information. In CDMA systems each cell site is assigned a unique Walsh cover, i.e., a Walsh code, which is used to encode information sent to that cell site. Therefore, only that cell site is able to decode the information. The selected cell site receives and decodes the CQI. It should be noted that the UE encodes its CQI and other signaling information with a particular type of additional coding that identifies the UE. All of the cell sites are provided with information about the additional coding used by the UEs that the cell sites service. Therefore, the selected cell site will be able to identify the UE from the type of additional coding the UE applied to the CQI and other information sent to the cell site. It should also be noted that cell switching encompasses the situation when a UE is switching from a sector of one cell to another sector of the same cell where the same is a portion of a cell that has its own sector cell equipment. Typically, the sectors use the same physical system equipment which is logically separated for each sector so as to make it seem as if each sector has its own system equipment.
The selected cell site then services the UE for at least a Transmission Time Interval (TTI). The TTI is a communication system defined time period during which signaling information and user information (i.e., data traffic) are transmitted from a cell site to a UE. The TTI for the UMTS system can be 10, 20, 40 or 80 milliseconds. The TTI for the evolving wireless data systems such as HSDPA in UMTS and 1xEV-DO is smaller than 10 milliseconds. The TTI in HSDPA is 2.0 milliseconds long and 1xEV-DO the TTI is 1.67 milliseconds long. Both the data traffic and the signaling information are carried in data blocks transmitted over a one or more TTIs. The user information or data traffic is information (e.g., voice, video, text, graphical information) being conveyed between users of the communication system. The signaling information is information used by the system to manage and otherwise control the operation of communication channels of the communication system. The signaling information usually complies with the protocol being followed by the communication system. The CQI is part of the signaling information.
Each cell site is limited in the amount of resources it can provide to the various UEs that it is servicing. When a UE selects a cell site for service, the UE bases such a selection on the CQI. However, the CQI may not be a proper indicator on the best cell site (in the active set of the UE) to select at that time. When a UE selects a cell site from its active set of cell sites because that cell site had the highest C/I or the highest data rate available in the active set of cell sites, the selection of that cell may not be—from an overall system point of view—the best cell site to select. The UE makes its decision based on its measurement of one particular channel quality of the members of its active set. Once a UE selects a cell site, the communication system then schedules the UE at the selected cell site. The system equipment that does the scheduling is commonly referred to as the scheduler. Thus, typically at any instant of time, a cell site has several UEs that are scheduled to be serviced. The scheduling is the determination of when a particular UE is to be serviced by a particular cell site. The UEs to be scheduled by the system are placed in a queue at the cell site which the UE selected. For each cell site, the scheduler then makes its scheduling decisions based on one or more system resource requirement of the UE which selected that cell site.
For example, suppose there are six user equipment, UE1, UE2, UE3, UE4, UE5 and UE6, all of whom can be serviced by either Cell-1 or Cell-2 of a CDMA communication system. In other words, Cell-1 and Cell-2 are cell sites that are members of the active sets of the six UEs. Also, suppose that UE1(3.0 Mb/s), UE2(10.0 Mb/s), UE3(8.0 Mb/s) selected Cell-1 and UE4(500 Kb/s), UE5(200 Kb/s), UE6(1.0 Mb/s) selected Cell-2 for service where the CQI on which the scheduler makes its scheduling decision is the data rate (value in parentheses) requested by each user. In other words, the scheduler selects the UE with the highest data rate from a queue of users for each cell site. The system scheduler places UE1, UE2 and UE3 in a queue at Cell-1 and UE4, UE5, UE6 in a queue at Cell-2. At the next scheduling, UE2 is scheduled on Cell-1 and UE6 is scheduled on Cell-2 because UE2 has the highest data rate of all the users in the queue for Cell-1 and UE6 has the highest data rate of all the users in the queue for Cell-2. The total data rate for the two cells is thus 11.0 Mb/s. The problem with the above scheduling scheme is that the scheduling decision is performed on a cell by cell basis without any consideration of the resources available from the overall system. In the example given above, it is possible that Cell-2 could have handled a higher data rate than the 1.0 Mb/s data rate of UE6, but because the UEs allocated to Cell-2 are the only UEs that can be considered in scheduling Cell-2, UE6 was the best choice at that time. Therefore, there is a lack of flexibility in scheduling which often leads to inefficient allocation of system resources.